In a conventional driving device for a three-phase brushless motor, the back electromotive forces V.sub.U, V.sub.V, and V.sub.W appearing in the coils of the motor are individually compared with the common-terminal voltage V.sub.N of the coils by a comparator to obtain three-phase square-wave signals P.sub.U, P.sub.V, and P.sub.W, which are then directly synthesized together to produce a speed signal (hereafter referred to as an "FG (frequency generator) signal") and drive signals D.sub.UU, D.sub.UL, D.sub.VU, D.sub.VL, D.sub.WU, and D.sub.WL.
The FG signal is then subjected to 30-degree phase shifting performed by a phase shifting circuit so as to be converted into a phase signal FGP. In accordance with this phase signal FGP and the above-mentioned drive signals D.sub.UU, D.sub.UL, D.sub.VU, D.sub.VL, D.sub.WU, and D.sub.WL, drive currents are supplied from a current supply circuit such as a power transistor circuit to the three-phase brushless motor.
However, in this conventional driving device, as long as the motor is rotating stably, the square-wave signals P.sub.U, P.sub.V, and P.sub.W exhibit stable waveforms as shown in FIG. 6; however, as shown in FIG. 7, when irregular variations occur in the back electromotive forces V.sub.U, V.sub.V, and V.sub.W as a result of polarity inversion caused by braking applied as in the period T, or by noise appearing as indicated by reference numerals 40 and 41, or by other factors, the square-wave signals P.sub.U, P.sub.V, and P.sub.W exhibit irregular variations accordingly.
Thus, the FG signal and the drive signals D.sub.UU, D.sub.UL, D.sub.VU, D.sub.VL, D.sub.WU, and D.sub.WL, which are produced from the square-wave signals P.sub.U, P.sub.V, and P.sub.W, are also affected such that the FG signal is no longer usable if left intact and that the drive signals D.sub.UU, D.sub.UL, D.sub.VU, D.sub.VL, D.sub.WU, and D.sub.WL cause unstable rotation of the motor, making it impossible to obtain stable rotation characteristics. Note that, in FIGS. 6 and 7, only the drive signals D.sub.UU, and D.sub.UL, are shown, and the other drive signals D.sub.VU, D.sub.VL, D.sub.UU, and D.sub.VL are omitted.